Conventional cases are generally made of wood and combined together with nails, and speed of their manufacture is slow, to a resultant high cost. Cases made of plastic shown in FIG. 4 are made of a long plastic plate cut with V-shaped grooves B, then folded up and glued together with connectors C, forming a complete case. Though this plastic case can be made quicker than wooden cases, and its material is easy to get, it needs adhesive process, which slows down manufacturing speed. Even if it is made by ejecting process, it still takes time to let it cool down for taking it off a mold, as it has a thick wall, not a hollow one, having heavy weight.